1. Field of the Invention
The present invention relates to a manual focus device and autofocus camera. More particularly, the present invention relates to a manual focus device and autofocus camera in which an in-focus state can be indicated and focusing can be easily effected.
2. Description Related to the Prior Art
Rangefinder incorporated cameras are well-known in the art of camera, in which a rangefinder of a double image coincidence type or a vertical image coincidence type is incorporated in a viewfinder. In the rangefinder incorporated camera, changes in a focused state due to focusing operation is observed as changes in a deviation between object images. Focusing with high precision is possible by observation with a human eye. The double image coincidence type of viewfinder has a view field where double images having an image through the viewfinder and an image through the rangefinder are observed. For focusing, the image through the rangefinder moving according to the focusing operation is adjusted and caused to coincide with the image through the viewfinder. The vertical image coincidence type of viewfinder uses a micro split image prism disposed in a light path in the viewfinder, and forms upper and lower split images of the object in a partial view field of the viewfinder. The upper and lower split images are moved horizontally in a symmetrical manner according to focusing operation. If an in-focus state is obtained, the split images coincide with one another.
There is a suggested structure of the viewfinder in JP-A 9-214813 and 2001-309210 in which an electronically photographed object image is utilized for focusing instead of incorporating the rangefinder for optically forming a double image or split images. According to this, double object images deviated according to a defocus amount are indicated with movement according to operation of focusing. It is possible to save and reduce a space and cost by reducing the number of parts for the optical structure.
For the purpose of the display of an image with movement in response to focusing and the display of coincidence of images at the time of the in-focus state, it is necessary to predetermine a defocus amount. According to the prior art, there are generally known methods including a triangulation method and a phase difference detection method. In the triangulation method, an object distance is measured by use of projecting and detecting infrared rays in order to calculate the defocus amount. In the phase difference detection method, two line sensors are used to receive object light through light paths symmetrical to each other with respect to the photographic optical axis, so as to calculate the defocus amount according to a phase difference between photoelectric signals from the line sensors.
Furthermore, an autofocus camera is well-known in which focusing is automatically effected. One example of autofocus camera includes an active type of rangefinder for projecting infrared rays. Another example of autofocus camera includes a passive type of rangefinder for utilizing object light. The rangefinder of any of those types operates according to the rangefinding of triangulation, in which an object distance is measured to obtain an in-focus position of a focusing lens for the purpose of focusing. Also, a digital still camera, video camera and other electronic equipment for photographing an image are known as the autofocus camera. A pickup element such as a CCD image sensor is utilized for picking up the object image. A contrast value is retrieved from the object image, and evaluated between pixels in each of frames. The focusing operation is automatically effected by obtaining a most highly sharpened contrast.
U.S. Pat. No. 6,453,124 (corresponding to JP-A 2001-281530) discloses the autofocus camera having a combined structure for focusing according to the contrast evaluation method and for focusing according to the phase difference detection method. In the phase difference detection method, the phase difference is detected between photoelectric signals of object light at points in two light paths which are symmetrical with respect to the photographic light axis, so an in-focus position can be determined by instantaneously obtaining an amount of defocus according to the phase difference. In this prior document, the lens is focused preliminarily according to the detection of the phase difference. After this, the lens is precisely focused by utilizing the evaluation of the contrast. This construction is effective in raising the speed of focusing operation without lowering the precision.
However, there occurs a high rise in the manufacturing cost for the construction to calculate the defocus amount according to the triangulation method. This is because of electric parts including the pickup element, a light projecting element for active rangefinding, a position detecting element and the like which require high precision in the incorporation. Similarly, the phase difference detection method requires a high manufacturing cost, because of an optical system for splitting and introducing object light, a line sensor, and other relevant elements.